The related background art known to applicant but which does not teach, disclose, or suggest the present invention includes separation by filtering, evaporation, settling, centrifugation, precipitation, distillation, adsorption, ion exchange, extraction, crystallization, chromatography, parametric pumping, and field flow fractionation.
Most of the above methods are fundamentally steady-state in character in the sense that continuous flow versions of the methods operate steadily in time after an initial transient period of start-up; in the absence of disturbances the state of the system becomes independent of time. In fact, control means are normally appended, an important object of which is the maintenance of steady product quality in the presence of external disturbances.
In contrast, the invention is a method of separation which is fundamentally periodic in character in the sense that the process always operates in the unsteady-state; it is fundamentally cyclic in its operation.
Also, most of the above methods are limited to one particular separative force. In contrast, the invention is a broadly applicable method of operation which can employ a variety of separative forces and can therefore be used on a wide variety of mixtures of materials.
The invention is most closely related to two of the above methods, parametric pumping and field flow fractionation.
In 1968, Wilhelm and co-workers introduced the parametric pumping process. Parametric pumping combines alternating fluid displacement in a straight column with an alternating, intensive, physical parameter such as temperature, pressure, or pH to separate molecular mixtures. For example, in thermal parametric pumping an alternating temperature causes adsorption and desorption of molecules within cross-sections on solid packing. Alternating fluid displacement, in one direction while the molecules are adsorbed, and in the other direction while the molecules are desorbed, taking advantage of compositional differences between adsorption and desorption equilibria, effects the separation. A closely related method called pressure-swing adsorption is used commercially to produce enriched oxygen from air.
Thus, parametric pumping employs two out-of-phase periodic actions: periodic variation of an intensive variable, typically temperature, and periodic forward and backward flow. The periodic variation of both the intensive variable and the flow are essential; without both, parametric pumping is inoperable.
The new invention differs from parametric pumping in that it uses a steady separative force rather than a periodic one, and specially modulated and controlled back-and-forth flows rather than symmetric periodic flows. The special back-and-forth flows bring about local periodic mixing and resuspension of the species being separated or fractionated, and also transport the species being separated along the conduit in which the process is being conducted. The local periodic mixing is enabled by a curved conduit, such as a helical tube, and/or flows which are asymmetric in their velocity. A species mixed or suspended in the fluid is transported by the motion of the fluid in one direction, and when the same species is adjacent to a solid boundary the other components are transported by the fluid in the opposite direction.
The invented method is usually orders-of-magnitude faster than, e.g. thermal parametric pumping because the invented method can use action-at-a-distance (body) forces as separative forces while thermal parametric pumping usually uses heat conduction and mass transfer, which are slow, diffusion limited processes. Each cycle in the invented method typically requires only a few seconds, while tens of minutes are needed typically for each cycle of thermal parametric pumping.
Field Flow Fractionating (FFF), a chromatographic method for fractionating macromolecules and fine particles, was conceived by J. C. Giddings in about 1965. A field acts perpendicular to flow through a narrow channel, forcing particles toward a wall. Simultaneously, diffusion tends to redisperse the particles. The particles least affected by the field, and most dispersed by diffusion in the cross section move downstream most quickly. A pulse of a mixture dispersed or dissolved in a fluid carrier which flows steadily through a tube will thus emerge in several fractions or peaks, as in chromatography. Giddings and others employed various types of external fields, yielding the following FFF subclasses: Sedimentation (centrifugal), Thermal, Thermogravitational, Flow (pressure gradient causes cross flow through membrane walls), Concentration, and Magnetic.
Sedimentation FFF is the version most closely related to the main example of the new invention presented shortly. In Sedimentation FFF, the flow channel is a helical tube within a centrifuge bowl. A rectangular cross section with a large aspect ratio is used to minimize secondary flow. Berg and Purcell developed a similar procedure prior to Giddings, but used a straight tube spinning on its axis rather than a helix. Sen has also recently used a spinning straight tube. About 1969, International Equipment Company (IEC), now Damon Industries, attempted to develop the "Helixtractor" unit for use with its centrifuges. The unit was intended for semi-continuous separation of red blood cells from blood; its principle of operation resembled Sedimentation FFF. Blood flowed unidirectionally through a helical tube in a centrifuge bowl. The object was to force red blood cells to the tube wall and collect them there in a semi-batch mode of operation, allowing passage of white blood cells. It was found, however, that unless axial flow rates were very small, that secondary flows resuspended the red blood cells, thwarting commercial practicality.
The new slurry fractionation method differs from Giddings' FFF in that rapidly alternating rather than fundamentally unidirectional flows are used, and in that the separated products are produced essentially in a steady fashion at opposite ends of the apparatus, while in FFF the products are produced in an essentially sequential series of pulses at one end of the apparatus. Also, periodic secondary flows are employed advantageously as an essential part of the new invention, rather than being suppressed as is critical to FFF. The new method can concentrate as well as separate fractions, while chromatographic methods such as FFF produce the separated species diluted in carrier fluid.
Examples of background art, but which differ from the present invention, are found in U.S. Pat. Nos. 3,449,938 and 4,147,621.